Multiple sclerosis (MS) is a demyelinating disease of the central nervous system (CNS) the cause of which remains as yet unknown.
Many studies have supported the hypothesis of a viral etiology of the disease, but none of the known viruses tested has proved to be the causal agent sought: a review of the viruses sought for several years in MS has been compiled by E. Norrby (1) and R. T. Johnson (2).
Concomitantly, the possibility of an exogenous and/or infectious factor is suggested by the existence of localized epidemics or "clusters" of MS, as have been observed in the Faro Islands between 1943 and 1960 (3), in Sardinia (4), in Norway (5), and also by studies on migrant populations (6). Among all the exogenous factors suggested, viruses have been most often studied, and a viral etiology is traditionally called to mind.
The observation in MS of phenomena which can be likened to an autoimmunity reaction has led to an "essential" autoimmune etiological hypothesis (7 and 8). However, this autoimmunity directed against certain components of the CNS has been found not to be specific to MS and common in inflammation of the CNS, whether or not associated with an infection (9, 10, 11 and 12). Furthermore, none of the immunosuppressive therapies has enabled decisive results to be obtained against MS (13). It now seems likely that the "autoimmune" manifestations are induced by a mechanism of viral origin: cosensitization to viral determinants associated with molecules of cellular origin, phenomena of molecular mimicry (14), or by expression of retroviral superantigens (15).
Some studies have supported a hypothesis according to which a retrovirus is at the origin of the disease: the recent discovery (16) of neurological syndromes associated with the HTLV-I virus, originally known as an adult T-cell leukemia agent, has led many authors (17, 18, 19, 20, 21, 22, 23) to look for an involvement of this human retrovirus in MS, however without success or with results suggesting cross-reactions.
Recently, a retrovirus different from the known human retroviruses has been isolated in patients suffering from MS (24, 25 and 26). The authors were also able to show that this retrovirus could be transmitted in vitro, that patients suffering from MS produced antibodies capable of recognizing proteins associated with the infection of leptomeningeal cells by this retrovirus, and that the expression of the latter could be strongly stimulated by the immediate-early genes of some herpesviruses (27).
All these results point to a role in MS of at least one unknown retrovirus or of a virus having reverse transcriptase activity which is detectable according to the method published by H. Perron (24) and qualified as "LM7-like RT" activity. The content of the publication identified by (24) is incorporated in the present description by reference.
Recently, the Applicant's studies have enabled two continuous cell lines infected with natural isolates originating from two different patients suffering from MS to be obtained by a culture method as described in the document WO-A-9320188, the content of which is incorporated in the present description by reference. These two lines, derived from human choroid plexus cells, designated LM7PC and PLI-2, were deposited with the ECACC on Jul. 22 1992 and Jan. 8 1993, respectively, under numbers 92072201 and 93010817, in accordance with the provisions of the Budapest Treaty. Moreover, the viral isolates possessing LM7-like RT activity were also deposited with the ECACC under the overall designation of "strains". The "strain" or isolate harbored by the PLI-2 line, designated POL-2, was deposited on Jul. 22 1992 under No. V92072202. The "strain" or isolate harbored by the LM7PC line, designated MS7PG, was deposited on Jan. 8 1993 under No. V93010816.
Starting from the cultures and isolates mentioned above, characterized by biological and morphological criteria, the next step was to endeavour to characterize the nucleic acid material associated with the viral particles produced in these cultures.